nematodes (Phylum Nematoda from Gr. nema, nematos
"thread" + ode "like") are one of the most common
phyla of animals, with over 20,000 different described species (over
15,000 are parasitic). They are ubiquitous in freshwater, marine,
and terrestrial environments, where they often outnumber other animals
in both individual and species counts, and are found in locations
as diverse as Antarctica and oceanic trenches. Further, there are
a great many parasitic forms, including pathogens in most plants
and animals, humans included. Only the Arthropoda are more diverse.

R
oundworms are triploblastic protostomes with a complete digestive
system. Roundworms have no circulatory or respiratory systems so
they use diffusion to breathe and for circulation of substances
around their body. They are thin and are round in cross section,
though they are actually bilaterally symmetric. Nematodes are one
of the simplest animal groups to have a complete digestive system,
with a separate orifice for food intake and waste excretion, a pattern
followed by all subsequent, more complex animals. The mouth is often
surrounded by various flaps or projections used in feeding and sensation.
The portion of the body past the anus or cloaca is called the "tail."
The epidermis secretes a layered cuticle made of keratin that protects
the body from drying out, from digestive juices, or from other harsh
environments, as well as in some forms sporting projections such
as cilia that aid in locomotion. Although this cuticle allows movement
and shape changes via a hydrostatic skeletal system, it is very
inelastic so does not allow the volume of the worm to increase.
Therefore, as the worm grows, it has to moult and form new cuticles.

Most free-living nematodes are microscopic, though a few parasitic
forms can grow to several meters in length. There are no circular
muscles, so the body can only undulate from side to side. Contact
with solid objects is necessary for locomotion; its thrashing motions
vary from mostly to completely ineffective at swimming.

Roundworms generally eat bacteria, algae, fungi and protozoans,
although some are filter feeders. Excretion is through a separate
excretory pore.

Reproduction is usually sexual. Males are usually smaller than
females (often very much smaller) and often have a characteristically
bent tail for holding the female for copulation. During copulation,
one or more chitinized spicules move out of the cloaca and are inserted
into genital pore of the female. Amoeboid sperm crawl along the
spicule into the female worm.

Eggs may be embryonated or unembryonated when passed by the female,
meaning that their fertilized eggs may not yet be developed. In
free-living roundworms, the eggs hatch into larva, which eventually
grow into adults; in parasitic roundworms, the life cycle is often
much more complicated.

Roundworms have a simple nervous system, with a main nerve cord
running along the ventral side. Sensory structures at the anterior
end are called amphids, while sensory structures at the posterior
end are called phasmids.

Parasitic forms often have quite complicated life cycles, moving
between several different hosts or locations in the host's body.
Infection occurs variously by eating uncooked meat with larvae in
it, by entrance into unprotected cuts or directly through the skin,
by transfer via blood-sucking insects, and so forth.

Nematodes commonly parasitic on humans include whipworms, hookworms,
pinworms, ascarids, and filarids. The species Trichinella spiralis,
commonly known as the trichina worm, occurs in rats, pigs, and humans,
and is responsible for the disease trichinosis. Baylisascaris usually
infests wild animals but can be deadly to humans as well. Haemonchus
contortus is one of the most abundant infectious agents in sheep
around the world, causing great economic damage to sheep farms.
In contrast, entomopathogenic nematodes parasitize insects and are
considered by humans to be beneficial.

One form of nematode is entirely dependent upon the wasps which
are the sole source of fig fertilization. They prey upon the wasps,
riding them from the ripe fig of the wasp's birth to the fig flower
of its death, where they kill the wasp, and their offspring await
the birth of the next generation of wasps as the fig ripens.

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